The present invention is a glass ceramic material and method of making, specifically for use in electrochemical devices such as fuel cells, gas sensors, oxygen or hydrogen pumps/separators, or for sealing any material with a thermal expansion coefficient similar to the seal material.
As used herein, the terms xe2x80x9csolid electrolytexe2x80x9d or xe2x80x9csolid oxide ion conducting electrolytexe2x80x9d are interchangeable.
As used herein, the term xe2x80x9cjointxe2x80x9d includes the term xe2x80x9csealxe2x80x9d because, in this glass-ceramic field, the xe2x80x9csealxe2x80x9d joins at least two parts. However, the xe2x80x9cjointxe2x80x9d may be intermittent thereby not serving as a xe2x80x9csealxe2x80x9d.
Ceramic materials are being used more often from automobile turbochargers to experimental fuel cells. However, there remains the problem of joining and/or sealing ceramic components to other ceramic components, to metal components, or to combinations thereof (e.g., cermet components) such that the joint maintains integrity during operation. For example, solid oxide ion conducting electrolytes are useful for oxygen separation and high temperature fuel cells. Although many technical challenges of their development have been overcome, there remains the problem of sealing. In a planar design, a gas-tight seal must bond the components together and prevent the mixing of the gas species on both sides of the solid oxide ion conducting electrolyte.
A limited number of materials are suitable as a solid oxide ion conducting electrolyte. The most commonly used materials are yttria stabilized zirconia (YSZ), doped ceria, doped bismuth oxide and doped lanthanum gallate. The thermal expansion coefficient of these materials can range from 10.1xc3x9710xe2x88x926 to 14.3xc3x9710xe2x88x926xc2x0 C.xe2x88x921 depending on the type of dopant and concentration. The operating temperature can also range from 700xc2x0 C. to 1000xc2x0 C. depending upon which material is chosen as the electrolyte. Therefore, the seal material must be tailored to match the electrolyte thermal expansion, maintain a gas tight seal at temperatures ranging from 200xc2x0 C. to 1200xc2x0 C., and not have detrimental chemical interactions with the fuel cell components. In addition, the seal material must also be stable at the operating temperature (800-1000xc2x0 C.) for extended periods of time ( greater than 9,000 hr) and be electrically insulating. For a solid oxide fuel cell, the seal must be able to survive extremely reducing environments.
Various efforts to seal solid oxide ion conducting devices have been made with varying degrees of success. Silica, boron, and phosphate-based glasses and glass-ceramics have been evaluated as a sealing material1-4 for solid oxide fuel cells. Experiments conducted by P. H. Larsen et all have shown major problems with glasses purely based on phosphate as the glass former. At temperature, the phosphate volatilized and reacted with the anode to form nickel phosphide and zirconiumoxyphosphate. Additionally, these phosphate glasses usually crystallized to form meta- or pyrophosphates, which exhibited low stability in a humidified fuel gas at the operating temperature.
Borosilicate glasses and glass ceramics have also been considered as potential seal materials. These glasses have been investigated by C. Gxc3xcnther et al2 and K. L. Ley et al3 for use in solid oxide fuel cells. However, boron will react with a humidified hydrogen atmosphere to form the gaseous species B2(OH)2 and B2(OH)3 at the operating temperature2. Therefore, any high boron seal may corrode in a humidified hydrogen environment over time. Glasses with B2O3 as the only glass former have showed up to a 20% weight loss in the humidified hydrogen environment and extensive interactions with fuel cell component materials both in air and wet fuel gas.1 
Silica-based glasses and glass-ceramics offer the most promise. They typically have a higher chemical resistance and show minimal interaction with the fuel cell component materials. Unfortunately, these glasses tend to have thermal expansions below the range needed for a sealing material.
At the operating temperature, most glasses will crystallize with time. Therefore, it is critical to have a glass composition in which the thermal expansion coefficient after crystallization is compatible with the solid oxide ion conducting electrolyte. Once the glass is fully crystallized, it is typically very stable over time. In addition, crystallized glasses tend to be stronger mechanically at operating temperature, improving seal performance.
Hence, there is a need in the art for a sealing material that can operate at an operating temperature up to about 900xc2x0 C., has a thermal expansion coefficient between 8xc3x9710xe2x88x926 and 15xc3x9710xe2x88x926xc2x0 C.xe2x88x921, and has no detrimental chemical interactions with the components.
1. P. H. Larsen, C. Bagger, M. Mogensen and J. G. Larsen, Proc. 4th Int. Symp. Solid Oxide Fuel Cells, Volume 95-1, 1995, pp. 69-78.
2. C. Gunther, G. Hofer and W. Kleinlein, Proc. 5th Int. Symp. Solid Oxide Fuel Cells, Volume 97-18, 1997, pp. 746-756.
3. K. L. Ley, M. Krumpelt, R. Kumar, J. H. Meiser, and I. Bloom, J. Mat. Res., Vol. 11, No. 6, (1996) pp. 1489-1493.
4. Yoshinori Sakaki, Masatoshi Hattori, Yoshimi Esaki, Satoshi Ohara, Takehisa Fukui, Kaseki Kodera, Yukio Kubo, Proc. 5th Int. Symp. Solid Oxide Fuel Cells, Volume 97-18, 1997, pp. 652-660.
The present invention is a glass-ceramic compound and method of making that are useful in joining or sealing ceramic components to other ceramic components, to metal components, or to combinations thereof (e.g., cermet components). More specifically, the present invention is useful for joining/sealing in an electrochemical cell having at least one solid electrolyte having a first and second side exposed to first and second gas species respectively. The seal is necessary for separating the first and second gas species.
The glass-ceramic compound contains at least three metal oxides, M1-M2-M3. M1 is BaO, SrO, CaO, MgO, or combinations thereof. M2 is Al2O3 and is present in the compound in an amount from 2 to 15 mol %. M3 is SiO2 with up to 50 mol % B2O3. The compound substantially matches a coefficient of thermal expansion of the solid ceramic component and at least one other solid component that is either ceramic, metal, or a combination thereof.
According to the present invention, a series of glass ceramics in the M1-Al2O3-M3 system can be used to join or seal both tubular and planar ceramic solid oxide fuel cells, oxygen electrolyzers, and membrane reactors for the production of syngas, commodity chemicals and other products.
It is an object of the present invention to provide a compound useful for joining or sealing a solid electrolyte or a solid oxide ion conducting electrolyte.
An advantage of a joint/seal made with the compound of M1-Al2O3-M3 is the maintaining of a substantially constant coefficient of thermal expansion from the glass to crystalline phase.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.